The relationship of molybdenum to health is far less well known by the general public than that of familiar essential metals iron, copper and zinc. Yet rare inherited diseases resulting in compromised molybdenum enzymes lead to pervasive neurological problems at best and are fatal at worst. Not only humans, but nearly every organism on this planet relies on one or more molybdenum or tungsten enzymes. Having such key roles for life at all levels of complexity, molybdenum enzymes are key participants in the global biogeochemical cycling of the elements carbon, nitrogen and sulfur. Despite huge progress made by the research community in understanding molybdenum and tungsten enzymes, very little is understood about the role of the special ligand common to both Mo and W enzymes. This ligand, nicknamed molybdopterin, is a pterin-substituted dithiolene chelate and is unique in all of biochemistry. While much has been learned about why a dithiolene chelate is suited for the electron transfer reactions catalyzed by the metals of these enzymes, the requirement of the pterin substituent remains largely a mystery. The new models that are the focus of this project incorporate a dithiolene chelate appended by a pterin, both of the key features of molybdopterin in all Mo and W enzymes, and therefore will be unique for their potential to study the gamut of pterin redox chemistry for a pterin-substituted dithiolene ligand coordinated to Mo. The proposed project seeks a detailed understanding of the unique aspects of the pterin-dithiolene structure that influence both the reactivity at molybdenum and within the coordinated pterin-dithiolene ligand itself. Specific objectives are: (a) explore the redox behavior of a molybdenum-coordinated pterin-dithiolene by chemical and electrochemical methods; (b) to probe the electronic effects of pterins at different levels of reduction on the Mo-dithiolene unit; (c) to accomplish full spectroscopic and structural characterization of all model compounds; (d) to begin an investigation of dithiolene transfer between molybdenum and copper. The results of the proposed work are needed to make further progress in understanding the function, and possibly the dysfunction, of the molybdenum enzymes in human metabolic processes. It is expected that studies of these models whose electronic structure closely resembles that of the molybdenum active site will: a) reveal the special purpose of the pterin in the active site of all molybdopterin enzymes; b) provide spectroscopic and structural benchmarks to aid interpretation of analogous results from the enzymes and c) provide examples of fundamental chemistry needed to understanding the active site chemistry of Mo and W enzymes, possibly laying groundwork for future therapies. The relationship of molybdenum to health is far less well known by the general public than that of familiar essential metals iron, copper and zinc, yet rare inherited diseases resulting in compromised molybdenum enzymes lead to pervasive neurological problems at best and are fatal at worst. More recently, a possible link between copper levels and molybdenum enzymes has been discovered. The goal of this project is to gain a more detailed understanding of how the unique ligand environment of molybdenum in molybdoenzymes is critical to the enzyme activity. [unreadable] [unreadable] [unreadable]